In radio transport networks a number of different technologies may be used, such as Long Term Evolution (LTE), LTE-Advanced, 3rd Generation Partnership Project (3GPP) Wideband Code Division Multiple Access (WCDMA), Global System for Mobile communications/Enhanced Data rate for GSM Evolution (GSM/EDGE), Worldwide Interoperability for Microwave Access (WiMax), or Ultra Mobile Broadband (UMB), just to mention a few possible implementations. A radio transport network comprises Radio Base Stations (RBS) providing radio coverage over at least one respective geographical area forming a cell. The cell definition may also incorporate frequency bands used for transmissions, which means that two different cells may cover the same geographical area but using different frequency bands. User equipments (UEs) are served in the cells by the respective radio base station and are communicating with a respective radio base station. The user equipments transmit data over an air or radio interface to the radio base stations in uplink (UL) transmissions and the radio base stations transmit data over an air or radio interface to the user equipments in downlink (DL) transmissions.
In some RBS implementations a radio unit and a baseband processing equipment (digital unit (DU)) of the RBS are separated. In some examples, the radio unit and baseband processing equipment is split in two different locations.
In this case, the radio unit is remote and termed a remote radio unit (RRU). As such, the system separates a RBS into one or more DUs and RRUs.
The DUs and RRUs are connected, for example, via a transport network, such as an optical transport network. The one or more DUs may be centralized and located remotely, for example a few kilometers from the RRUs. The RRUs are placed close to the radio antennas, e.g. in antenna masts. This minimizes feeder and jumper losses between antenna and RRUs, which is often a major challenge to address in most radio transport networks, for example, to enhance the uplink capacity of mobile services.
Centralized radio access network (RAN) is based on the DU and signal processing centralization, which offers processing resources for multiple cells, each covered by an antenna driven by a RRU. This allows a pool of processing resources to be dynamically shared among many cells, in line with the cloud computing principle, saving energy, improving the radio link reliability and decreasing the number and size of access sites. Centralized DU processing can therefore help reduce the cost of the infrastructure and favor coordination among different RRU pools.
In systems such as Long Term Evolution (LTE) and LTE Advanced (LTE-A), where coordinated processing is beneficial to performance improvements, the capability to manage this centrally rather than via an external X2 interface between base stations could generate important performance gains.
In addition, coordinated processing can be beneficial to reduce and manage inter-cell interference between neighboring cells and across access layers in heterogeneous networks where small cells are used to offload part of the traffic originally handled by a macro cell. Here, coordination is needed to avoid interference and to enable frequency reuse among macro and small cells.
In some examples, the interface between the DUs and RRUs is an optical Non-Return to Zero (NRZ) signal, which is a sampled In-phase Quadrature (I/Q) air interface waveform. Sampling the air waveform makes the remote radio unit implementation relatively simple but leads to very high bitrates of the optical signal, in the order of 1.25 Gbps per antenna.
A Common Public Radio Interface (CPRI) specifies a Time Division Multiplexing (TDM) like protocol for RBS configurations in a system configured for RRUs and DUs over a first layer. CPRI defines a protocol which can be used to connect a DU and RRU. The application of CPRI between the DUs and the RRUs may be static, i.e. determined as the RBS is deployed, and in such an example the CPRI configuration is typically only changed as part of a predetermined topology involving the DUs and RRUs.
CPRI requires accurate synchronization and latency control. Even if conventional CPRI transport is normally operated on fiber using point-to-point optical connections between DU and RRU distances of less than a few hundreds of meters, there is a demand to extend its reach over geographical distances.
Traditional dedicated point-to-point links established between a limited number of DU ports and associated RRUs is inadequate to meet these new extended distance requirements. For example, the requirements of new installed fibers would be not sustainable as soon as the distances between RRUs and associated DUs became longer than a few kilometers.
Moreover, an increase of the optimization level could be achieved by having a pool of DUs serving a plurality of RRUs. This allows a wider geographical area, enabling a higher optimization of computational resources. The use of a pool of DUs should deal with at least the following issues:                CPRI has tight requirements and constrains (e.g. latency, jitter, symmetry). A transport and switching solution should therefore comply with them;        Efficient load balancing and failure recovery requires the ability to change which DU(s) is handling an RRU, possibly without traffic disruption;        A consistent reduction of infrastructure cost requires optimizing the geographical cabling which supports the connectivity among RRUs and DUs.        
A transport network for providing connectivity among RRU clusters and a DU pool, as described above, is sometimes referred to as a front haul (FH) network. FH networks may use WDM (Wavelength Division Multiplexing) networking solutions, for example using an optical ring topology. The use of a FH network should not impact the quality of service of a radio network, and thus in the case of a failure, the FH network should be recovered without impacting on the radio service. In addition, the FH network should preferably provide support for differentiated classes of services.